The integrated circuit fabrication industry relies on the continual and repeated inspection of the integrated circuits as they are produced to ensure that processes are operating properly and that the integrated circuits themselves are properly formed. Such inspections can be generally divided into two different categories, being inline inspections and final inspections. Inline inspections are performed during the fabrication process, such as just after a critical step in the process is performed. In this manner, information can be fed back to the process in a timely manner, to ensure that subsequent products in the pipeline are not processed incorrectly. Final inspections are typically performed at the end of the fabrication process, and although they too may provide some valuable information in regard to the proper operation and control of the fabrication processes, they are primarily designed to ensure that the finished integrated circuits are functional and free of defects prior to packaging and sale.
Years ago, inspection steps were performed manually, and trained inspectors loaded a wafer on which the integrated circuits are formed under a microscope, and carefully scanned across the surface of the wafer, looking for defects. This system, of course, had many drawbacks. For example, different operators would interpret different wafer characteristics as defects, and thus there was a problem with inspection uniformity. In addition, even a well trained operator may have a momentary lapse of attention, and miss an important defect. Finally, even under the best of conditions, manual inspection for defects became too time consuming.
For these and other reasons, automated inspection became the preferred method of inspecting integrated circuits. Automated inspection solved the problem of non-uniformity between different inspectors, because the automated inspection tools could run from the same software routines. Further, automated inspection tools do not have momentary lapses in the same manner that human inspectors did. Finally, for the most part, automated inspection tools were much faster than their human counterparts. Thus, automated inspection has traditionally worked well for the integrated circuit fabrication industry.
However, as automated inspection has evolved in the wake of ever more complicated integrated circuits, there has been continual pressure to detect and classify a growing number of defects and other conditions that may be present on the wafer. Thus, a heavier load has been placed on automated inspection than was ever conceivable for to manual inspection. This situation is amplified by the fact that fabs are producing more and more integrated circuits so as to reduce the overhead costs associated with production, and to satisfy the demands of the modern world. Such conditions have resulted in a desire to reduce the time associated with automated inspection.
What is needed, therefore, is a system by which the time required to inspect a wafer can be reduced without unduly reducing the accuracy of the inspection.